Plant for tempering of a building

ABSTRACT

A plant for tempering a building, including an energy storage unit formed by ground heat exchangers coupled in parallel and each including a heat insulated leg and a non-insulated leg in thermal contact with surrounding soil. The heat exchangers are included in a circulating circuit for a circulation fluid which includes a circulation pump and room temperature keeping devices. The energy storage unit has a continuously varying temperature in a depth direction from a cold end to a warm end. The flow direction in the ground heat exchangers is controlled by a reversing valve such that for a warm keeping function, circulation fluid is received from the warm end and for a cool keeping function, circulation fluid is received from the cold end. The circulation pump is controlled such that the circulation flow brings about a temperature difference between the inlet and outlet of the ground heat exchangers that is constant with a value that can be calculated with the aid of the properties of the ground and the actual components.

This application is a U.S. National Phase Application under 35 USC 371of International Application PCT/SE03/00167 filed Jan. 31, 2003.

The present invention relates to a plant for tempering a building thatincludes: an in-ground energy storage unit formed by a plurality ofground heat exchangers coupled in parallel, each ground heat exchangerincluding a first tube-shaped leg surrounded by heat insulation and asecond leg in thermal connection with surrounding soil; at least oneroom temperature keeping device for at least one room; and a circulationpump for circulating circulation fluid through a circulation circuit,which includes the circulation pump, the first and second legs of theground heat exchangers, and the at least one room temperature keepingdevice, and in which the circulation fluid flows down one leg and up theother leg in each of the ground heat exchangers.

The plant comprises mainly a combination of two very special knowncomponents. A first such component is the ground heat exchangers, in thefollowing named GHEX, which are of the type described in e.g. theSwedish patent specifications 408 087, 450 289, 458 061 and 513 218.They have a first tubular leg provided with heat insulation, and asecond leg in close thermal contact with surrounding soil. Due to thefact that the two legs are in a very bad mutual thermal contact, and theleg without heat insulation is in very god thermal contact with thesurrounding soil, the heat exchangers will become extremely efficientand make it possible to obtain a stratification of the temperature inthe energy storage. An energy storage comprising a number of ground heatexchangers can be loaded in summer time by conducting a warm circulationfluid down and up through the two legs of each ground heat exchanger andfurther to a circulation circuit comprising room temperature keepingdevices, which are the above mentioned second known component in thebuilding, in the following named RTKD. These devices can be of the typedescribed in the Swedish patent specifications 442 132, 441 535 and 460731, in the following named RTKD-1, or conventional counter-current heatexchangers, in the following named RTKD-2, which latter are suitable fortempering ventilation air by means of the circulation fluid in thecirculation circuit. The room temperature keeping devices known from thepatent specifications are intended to be integrated with the outer wallstructure of buildings and to bring about a balancing of thetransmission losses in the outer walls. The arrangement makes itpossible to keep rooms warm at cold ambient temperature, e.g. in wintertime, i.e. a direct compensating of transmission losses, with only a fewdegrees higher forward feeding temperature than the room temperature,and in that case a return temperature considerably below the roomtemperature. The arrangement gives rise, in a corresponding way, at highambient temperature, e.g. in summer time, to a cool keeping of roomswith only a few degrees lower forward feeding temperature than the roomtemperature and in that case a return temperature considerably above theroom temperature.

A conventional ground heat energy storage is drained of energy when warmfluid flows from the storage and is replaced by colder fluid. To extendthe time for the decay of the storage and to secure that sufficient ofenergy is left at the end of the winter season, if a late cold periodshould occur, it might be necessary to occasionally supply heat to theflow of fluid returned to the storage.

In many cases, and especially in countries with a ground temperatureamounting up to 20° C., is it known e.g. from the above-mentionedSwedish patent specification 408 087 to alternatively arrange, by meansof the above described method, a cooler ground body with a temperatureof about 10-15° C. for keeping a building cool, which building issubject to a long term supply of undesired heat.

Thus, if both warm keeping and cool keeping is required, two mutuallydistant ground bodies are necessary, which involves a great deal ofspace, and bad operating economy as well as high initial expenses.

The object of the invention is among others to achieve a realimprovement in a plant of the type set forth in the introduction, whichin a simple way secures that the plant can be used both for warm keepingand cool keeping of the interior of a building by using to the greatestpossible extent low quality energy, i.e. natural energy.

This is achieved according to the invention by a plant having thecharacterizing features of the following claims. Due to the fact thatthe plant has been built-up by a combination of the two very specialabove-mentioned components, and the plant is designed to be operated insuch a way that, partly the flow quantity of the circulation fluid inthe ground heat exchangers is adjusted such that a great and concistentdifference is achieved between forward feeding temperature and returntemperatur of the circulation fluid, partly the circulation direction inthe ground heat exchangers is different at warm and cool keeping, resp.,of the buildings, an energy storage is achieved in the ground withdownwards stratified temperature and forming a warm and a cold end.

It is possible in principle both to have the warm end at the top and thecold end at the bottom and vice versa. It is especially suitable,however, to locate the cold end at the bottom with the three-dimensionalthermal leakage directed towards the surrounding cold ground, in whichcase the warm end will be located between the cold end and the groundsurface. The opposite may be the case in very warm countries where it isnecessary to preserve cooling temperatures which may differ a great dealfrom the natural ground temperature. Hence, the cold end intended forcooling purpose should not be located in the bottom area, sensitive forthermal leakage. In some of such countries the nights are very cold,which makes it easy to collect coldness during the night and heat duringthe day, at the same time as warm keeping and cool keeping, resp., iscarried out after only changing of the fluid flow direction.

The invention will be more completely described in the following withreference to the accompanying drawings, which schematically disclose anexample of the realizing of the invention, and in which

FIG. 1 a is a sectional view of a first example of a temperature keepingdevice (RTKD-1) for balancing heat transmission losses with associatedtemperature profiles for winter conditions, to the left on the drawing,and sommer conditions, to the right on the drawing;

FIG. 1 b is a corresponding presentation as in FIG. 1 a of the secondtype of room temperature keeping device (RTKD-2), which is a typicalcounter-current heat exchanger for tempering of the ventilation air;

FIGS. 2 a and 2 b is a cross sectional view of a ground heat exchanger,and a longitudinal sectional view, resp., of a part of the heatexchanger;

FIG. 3 is a graph showing the temperature profile of the ground bulk atvarious time periods of the year;

FIG. 4 is curves showing the time development during a year of, in FIG.4 a the heat transmission power per meter for a ground heat exchanger,in FIG. 4 b the magnitude of the thermal energy transport between thecirculation fluid and the driving temperature difference betweencirculation fluid and the ground bulk at a given ground depth,controlling the ground, in FIG. 4 c the magnitude of the temperaturevariations around the annual mean value at the ground depth in questionfor the ground bulk temperature and the circulation fluid temperature inthe non-insulated leg, and in FIG. 4 d the magnitude of the temperatureof the circulation fluid comng up from the energy storage;

FIG. 5 a is a graph showing the temperature profile of the circulationfluid and the ground at maximum load in the middle of the winter andsummer, resp. as a first approximation, and FIG. 5 b a graph showing thereal shape of the temperature profiles in FIG. 5 a;

FIG. 6 is a diagram chosen by example, of a simple plant according tothe invention with, to the right on the drawing, the temperaturstructure shown for two types of room temperature keeping devices andfor the flow in one of the ground heat exchangers of the energy storageduring winter condition; and

FIG. 7 is a diagram of the same plant as shown in FIG. 6 during summercondition and with temperature structures changed with respect thereto.

The drawings show throughout circumstances during summer and winterperiods, but the circumstances are in principle similar at shorterperiods, e.g. at periodic, in other ways varying ambient temperatures,as during night and day, which in certain geographic areas can varyconsiderably.

FIG. 1 a shows a room temperature keeping device RTKD-1 with aninsulated outer wall 10, an inner wall 11 and further an inner channel12 and an outer channel 13 which are separated by a heat insulatedpartition wall 14. A circulation fluid CF flows through the innerchannel bows back at the top and flows out through the outer channel 13.It is achieved, as is clearly described in in the above mentionedpatents, and especially in 460 731, 442132 and 441 535, with thecirculation fluid consisting of liquid exchanged to circulating air inchannels as shown above via a counter-current heat exchanger, a warmkeeping function in winter time with only a few degrees higher forwardfeeding temperature than the room temperature, and in that case a returnfeeding temperature is achieved that is much below the room temperature.In an analogous way a cool keeping is achieved in the summer time withonly one or a few degrees lower forward feeding temperature than theroom temperature at the same time as the return feeding temperature isconciderably above the room temperature and the forward feedingtemperature. In the main reversed temperature profiles are achieved withthe room temperature keeping device RTKD-1, which profiles are shown tothe right on the drawing, FIG. 1 a, for winter and summer conditions.The temperature profile for the fluid in the inner channel 12 isindicated 12′ and 12″, resp., and the temperature profile for the fluidin the outer channel 13 is indicated 13′ and 13″, resp., for winter andsummer conditions, resp.

FIG. 1 b shows the corresponding temperature profiles as FIG. 1 a, burfor a room temperature keeping device RTK-2 in the shape of acounter-current heat exchanger, in which heat exchange is obtainedbetween the circulation fluid CF in a channel 15 and a flow of exteriorair VA in a channel 16 for room tempering of the ventilation air. Thetemperature profile for the fluid in the channel 15 is indicated 15′ and15″, resp., and the temperature profile for the air in the channel 16 isindicated 16′ and 16″, resp., for winter and summer conditions, resp.

The characteristic of the two types of room temperature keeping devicesare quite analogous with respect to inlet and outlet temperatures.

The ground heat exchanger GHEX shown in FIGS. 2 a and 2 b are of asimilar type as the one shown in the above mentioned Swedish patentspecification 513 218 and comprises a central tube 20 provided with anexterior heat insulating casing 21. Outside the casing 21 and at adistance from it a sleeve 22 is located in an essentially vertical borehole in close thermal contact with surrounding soil 23. The sleeve 22 isprovided with a great number of axial channels 24 tightly distributedaround the circumference of the sleeve. The tube 20 and the channels 24are the insulated and non-insulated legs, resp., of the ground heatexchanger.

FIG. 3 shows schematically the temperature profile of a ground bulkthermally coupled to a ground heat exchanger at different times of theyear, wherein the line 30 indicates the ground surface, which above theground bulk usually is covered by a heat insulating plate or building(not shown). The bottom of the energy storage is indicated by a line 31.Between the lines three sloping lines extend, which are essentiallyparallel. The lines 32,33 and 34 indicate the temperature profile atspring, summer, winter and autumn, resp. The temperature profiles showthat the temperature profile of the ground will be displaced in parallelwith itself during the year and reciprocate between two turningpositions, the lines 34,32 representing the state of completely fullloaded and completely unloaded energy storage.

FIGS. 4 a-d show course of events by time over the year for differentthermal parameters connected to the temperature keeping devices RTKD andground heat exchangers GHEX. The time concepts spring, summer, autumnand winter are marked along the horizontal time axes. FIG. 4 a shows acurve 40 similar to a sinus curve, which represents the heat transferpower per meter, W/m, of a ground heat exchanger GHEX. The loading has amaximum during the summer and the unloading has a maximum during thewinter. The heat transfer power of the ground heat exchanger is createdby a “driving” temperature difference between the circulation fluid inthe leg 24 of the ground heat exchanger and the bulk of the ground. Thistemperature difference at a given depth in the ground is represented bythe curve 41 in FIG. 4 b and has maxima at the same points of time asthe curve 40 in FIG. 4 a.

The temperature variations round a mean annual value is illustrated byFIG. 4 c, in which a full curve 42, which shows the temperaturevariations of the circulation fluid in the non-insulated leg 24, and adashed curve 43 shows the temperature variations of the ground bulk atthe actual depth in the ground.

In FIG. 4 d a dashed curve 44 shows the necessary forward feedingtemperature of the circulation fluid during a year. The temperature inthe non-insulated leg 24 at the surface of the ground then will berepresented by the full curve 45, and the temperature of the insulatedleg 20 both at the bottom and at the surface will be represented by thefull curve 46.

The circulation fluid flows up from the insulated leg 20 during thesummer and during the winter from the non-insulated leg 24, as isschematically indicated below FIGS. 4 a-d, which means that thecirculation fluid that flows up from the energy storage has atemperature that follows the curve 47 indicated by double lines in FIG.4 d.

From FIG. 4 a is evident, that the maximum heat transfer power atloading as well as unloading occurs when the ground has its mean annualtemperature at the actual depth. This means that the operationconditions of the ground heat exchanger at the two significant anddimensioning occasions of the year when maximum power should betransfered to or from, resp., the ground can be illustrated by a singlediagram shown in FIG. 5 a. The circulation direction of a ground heatexchanger GHEX is different at transfer of energy to and from, resp.,the ground—i.e. summer and winter, resp.,—and that the outlet from theground heat exchanger—i.e. delivery of energy—takes place from differentlegs summer and winter, resp. At maximum loading and unloading at eachdepth it will be necessary to have a driving temperature differencebetween the circulation fluid in the non-insulated leg and the groundbulk. The magnitude of this depends on the thermal resistance anddesired heat power. The necessary driving temperature difference can bevery low at ground heat exchangers of the type discussed here, only oneor a few degrees. The driving temperature difference is at each timemoment essentially the same at each depth, and, thus, delivers the samepower per meter of the depth of the ground heat exchanger. Hence, alinear change of of the temperature of the circulation fluid in thenon-insulated leg is achievd, but the flow in the thermal insulated leghas a constant temperature. In FIG. 5 a the line 50 shows thetemperature in the middle of the summer of the circulation fluid in thenon-insulated leg from the top of the energy storage to its bottom wherethe fluid flows over to the insulated leg with a constant temperatureindicated by a vertical line 51. The circlation fluid is fed down in themiddle of the winter in the insulated leg, in which the temperature isconstant and represented by a vertical line 52. At the bottom of the legthe fluid is transferred to the non-insulated leg in which thetemperature increases along a line 53. The ground temperature in themiddle of the summer as well as the winter follows a dashed line 54.

FIG. 5 a is an approximation which for example does not considervertical conduction in the ground and that the insulated leg of theground heat exchanger of course cannot have a perfect thermalinsulation. The true courses and shapes of the temperature profiles areas shown in FIG. 5 b, where the straight lines 50-53 in FIG. 5 a areshown somewhat curved in FIG. 5 b and are denoted 50′-53′. Thetemperature profile of the ground is here denoted 54′ and has a somewhatchanged slope as compared with curve 54 i FIG. 5 a.

From FIG. 5 a and FIG. 5 b is evident, that the energy that has to becollected for warm keeping, a half year later has a temperature range55, and energy that has to be collected for cool keeping, a half yearlater has a temperature range 56. These temperature ranges overlap eachother to a great extent and both include the intended room temperaturewhich is indicated by a line 57.

The observation of these surprising and remarkable facts lays thefoundation of the possibility of collecting natural energy by simplemeans and makes it possible to to create very robust and economicalplants for tempering of buildings.

By the combination of components and operating strategy here shown, anenergy storage is created with a comparatively warm and cold end. It ispossible in principle to project a system with th warm end either in thebottom or at the ground surface, but the total heat leakage from thestorage usually becomes lower with the warm end at the top and the coldend at the bottom where the leakage pattern is three-dimensional, whichconsequently makes the heat leakage sensitive to great temperaturedifferens between the storage and the natural temperature of the ground.The cold end of the storage has a temperature, which at least inScandinavian zones is rather close to the natural temperature of theground, and therefore the thermal leakage becomes relatively small.

It may possibly be meaningful in warm countries to locate the warm endin the bottom because it is important to take care of the coolingtemperature, which can be far from the natural temperature of theground, which means that the cold end for cool keeping shall not belocated in the bottom zone sensitive to thermal leakage.

The upper end of the storage, whether warm or cold, is usually thermallyinsulated by an insulating layer or a building on top of the storage.

Demands for the design of utilized components and operating conditionsthat have to be fulfilled for meeting the thermal part of the demand ontotal energy needs for tempering a room by only natural energy, can besummarized by the following:

A plant comprises in the first place a combination of necessarycomponents like room temperature keeping devices and ground heatexchangers. The plant is operated with such an operating strategy thatthe circulation direction is different during summer and winter, andthat the circulation flow is adjusted such, that the temperaturedifference S between the inlet to and outlet from the ground heatexchanger always is kept at a constant value, that is at least 50%,preferably about 100% of the value S_(min) for a given plant, whichvalue is equal to F+U+L, where:

-   -   F=the temperature difference between necessary forward feeding        temperature to the room temperature keeping devices at maximum        warm keeping power at cold ambient temperature and at maximum        cool keeping power at warm ambient temperature;    -   U=necessary driving temperature difference at the ground surface        between the ground and the circulation fluid in the        non-insulated leg at maximum heat power outlet from the ground        at cold ambient temperature;    -   L=necessary driving temperature difference at the ground surface        between the ground and the circulation fluid in the        non-insulated leg at maximum cool power outlet from the ground        at warm ambient temperature.

Hence, the quantity S_(min) above is a fixed quantity that is settled atthe projecting and dimensioning of a plant and where F is decided by theproperties of the utilized RTKD components. The two quantities U and Lare decided by the properties of the GHEX components and the ground.

The control signal that controls the flow of the pump 65 consists of thediffering from actual, i.e. continuously measured, temperaturedifference S from the wanted constant value S_(min). At too large valueS the flow will increase so that S decreases and vice versa according toknown art. This means that the temperature difference S, independent ofthe load conditions of the plant, will be adjusted close to the wantedvalue Smin.

This operating strategy leads to the formation of a warm end and a coldend of the energy storage (compare FIG. 5 b), which lays the foundationof the following unique distinguishing feature:

The warm end of the storage has a sufficiently high temperature for warmkeeping during winter time, i.e. at cold ambient temperature, by meansof room temperature keeping devices, RTKD, but low enough to permit theuse of heat energy of very low energy quality for loading of heat to theenergy storage. At cool keeping—summer time—the room temperature keepingdevices, RTKD, will supply cool energy with a temperature that is higherthan the room temperature, which results in that this component works asa heat energy collector. If this energy in some cases should beinsufficient, is it easy to to collect complementary energy by simplemeans. Even simple solar collectors without glazing are effective onthese conditions for collecting energy with high efficiency. Collectionof energy for warm keeping may also be effected from low-quality, andthus cheap, waste heat.

Further, the cold end of the energy storage has a sufficiently lowtemperature for cool keeping by means of room temperature keepingdevices, RTKD, but, however, so high that cooling energy easily can becollected in winter time by means of room temperature keeping device,RKTD, if necessary completed with special collectors for low-qualityenergy, which may be the case at places with a very great need of coolkeeping. At places with inland climate or high mean annual temperaturecool collection takes place in the winter and in the night, resp., whichcan be carried out by exposure of surfaces of colletcors for low-qualityenergy to clear and cold night sky.

In all the above reasoning it is presumed that all the need of thermalenergy for tempering should be received via the energy storage in groundthat is loaded with natural energy, i.e. no high-quality energy in stateof energy generated by equipment for temporary energy production isnecessary at normal operation. It is for different reasons possible toconsider in certain situations to quite deliberately refrain fromreaching this limit for complete energy autonomy. One may be contentwith only a partial use of the potential of the system.

To illustrate what unconventional temperatures is all about, anexemplification is given in the following in connection to FIG. 6 andFIG. 7, which schematically show a simple plant according to theinvention at winter operation in FIG. 6, and at summer operation in FIG.7, and with the temperature structure for the different components shownto the right in the figures and on a level with the respectivecomponents. The exemplification is definitely no optimization, but theobject is to illustrate the fundamental opportunities to utilize aground heat storage with a warm and a cold end. The exemplificationillustrates a dimensioning so adapted that no additional collecting ofcoolness in the winter is necessary with a collecting device forlow-quality energy. A device of this type is necessary only in thesummer. e.g. a simple solar collector, for completion of the collectingof heat that is made to 25° C. by room temperature keeping devices. Noshunting is used in the two shown operating conditions, because theserepresent dimensioning maximum load for warm keeping and cool keeping,resp. At partial load shunting with a valve 74 is necessary for reachinga suitable forward feeding temperature to the room temperature keepingdevices. The dimensioning example is further adapted, to elucidate thefundamental principles, for such a system size that the heat leakagebecomes negligible.

FIG. 6 discloses one of several in parallel connected ground heatexchangers GHEX with a heat insulated leg 20 and a non-insulated leg 24,as described above. The leg 24 is by a pipe 60 connected to an equipmentfor temporary energy SUES (Supplementary Energy Supply), which by a pipe61 is connected to a collecting pipe 62. The leg 20 is by a pipe 63connected to a device for collecting low-quality energy LEEC (Low ExergyEnergy Collector), which by a pipe 64 is connected to a circulation pump65 having its suction side connected to a second collecting pipe 66.

To the collecting pipes 62,66 are, partly a previously described roomtemperature keeping device RTKD-1 connected with a forward feeding pipe70 and a return pipe 71, partly a previously described second roomtemperature keeping device RTKD-2 connected with a forward feeding pipe72 to the channel 15 for the circulation fluid and a return pipe 73 fromthe channel 15. Exterior air in counter-current is fed through thechannel 16 for tempering. An adjustable shunt 74 to the pipe 61 isconnected to the outlet of the circulation pump 65, and at the inlet tothe device LEEC is a second adjustable shunt 75 arranged connected tothe pipe 63. To the pipes 60,63 of the ground heat exchanger GHEX isfurther a reversing valve 76 arranged for reversing of the terminals tothe pipes 60,63.

For controlling and adjusting said components there is an informationprocessor 77, which by signal lines 78.79 sense the temperature of theinlet and outlet fluids through the pipes 60,63. The informationprocessor 77 can by signal lines 80,81 and 82 control connection anddisconnection of the equipment for temporary complementary energy SUES,control the flow of the circulation pump 65, and possibly requisite flowthrough the shunt valve 74 (not shown). Besides, there is of course alsofurther conventional control equipment, like thermostats for adjustingthe room temperature in different rooms and for controlling the shuntvalve 75 and so on (not shown).

In this example the basic temperature differences have been calculatedtoF=6KU=2KL=2K Smin=10K

The temperatures at the different components are then:

in ° C. out ° C. Winter time: a) RTKD (warm keeping + ventilation airtemp.) 24 14 b) GHEX (outlet of heat + loading of coldness) 14 24 c)LEEC (possible collecting of coldness) — — Summer time: a) RTKD (coolkeeping + ventilation air temp.) 18 25 b) GHEX (heta loading + coldnessout from ground) 28 18 c) LEEC (collecting heat) 25 28

FIG. 6, which relates to winter operation, discloses the temperaturecurve 90 of the ventilation air, and the temperature curve 91 of thecirculation fluid in the device RTKD-2. Further, the temperature curve92 for the inner channel 12 and the temperature curve 93 for the outerchannel 13 in the device RTKD-1 is shown. At last the temperature curve94 is shown for the non-insulated leg 24 in the ground heat exchangerGHEX and the temperature curve 95 for the ground.

FIG. 7, which relates to summer operation discloses the correspondingcurves as in FIG. 6 indicated with the same numerals provided with‘-sign. Here the reversing valve 76 at the forward and return pipes ofthe ground heat exchanger GHEX has been activated, e.g. by a device forsensing the return temperature from the building (not shown) forconnection of the pipe 63, which is connected to the insulated leg, tothe collecting pipe 62.

It has been possible, by combining above all the above-mentionedcomponents and utilizing the above outlined operating strategy, to coverthe thermal need for tempering of the interior of a building as well asthe ventilation air all the year round with only natural energycollected locally on the spot. Energy of higher quality, service energy,i.e. energy for control equipment, circulation pumps etc., must beadded, but constitutes only a marginal part.

The plant according to the invention has many advantages. Especially thefollowing may be pointed out:

Said components can, thanks to the extremely low operating temperatures,be given a simplified design but still get much better performance thanconventional components in priorly known systems. This holds good fore.g. solar collectors, which here shall be unglazed for highestefficiency.

One and the same investment gives both a warm keeping and cool keppingfunction with practically no working expenses. This is quite commonlypreferable, but in particular for buildings in places with inlandclimate, where the energy need is very large for both warm keeping andcool keeping, and in some of these regions it may be a question ofcollecting heat by day and coldness by night, at the same time astempering of the building is carried out all the day and night.

It is especially important that warm keeping in the winter can beobtained at the same time as cool energy is collected and stored in thecold end of the energy storage, and that cool keeping in the summer canbe obtained at the same time as heat energy is collected and stored inthe warm end of the energy storage.

The invention is of course not restricted to the embodiments here shownand described but can be modified in different ways within the frame ofthe invention defined in the claims. This is a qustion of for examplethe circulation fluid, which mostly is a liquid, but in room temperaturekeeping devices incorporated in outer walls as a matter of precaution isin the form of air, where the change between liquid and air is carriedout in a known manner by an efficient counter-current heat exchanger.

1. A plant for tempering a building, comprising: an in-ground energystorage unit formed by a plurality of ground heat exchangers coupled inparallel, each ground heat exchanger including a first tube-shaped legsurrounded by heat insulation and a second leg in thermal connectionwith surrounding soil, the energy storage unit having a continuouslyvarying temperature in a depth direction between a cold end and a warmend; at least one room temperature keeping device for at least one room;a circulation pump for circulating circulation fluid through acirculation circuit, which includes the circulation pump, the first andsecond legs of the ground heat exchangers, and the at least one roomtemperature keeping device, and in which the circulation fluid flowsdown one leg and up the other leg in each of the ground heat exchangers;and a valve device for controlling a flow direction of the circulationfluid in the ground heat exchangers such that when keeping a warmtemperature in the at least one room, circulation fluid is directed tothe at least one room temperature keeping device from the warm end ofthe energy storage unit and when keeping a cool temperature in the atleast one room, circulation fluid is directed to the at least one roomtemperature keeping device from the cold end, wherein the circulationpump is controlled such that a temperature difference S between an inletand an outlet of the ground heat exchangers at all operating conditionshas a constant value equal to at least 50% of a fixed value S_(MIN) forthe plant, wherein S _(MIN) =F+U+L where: F is a temperature differencebetween a required feeding temperature to the at least one roomtemperature keeping device at a maximum warm keeping power at coolambient temperature and at maximum cool keeping power at warm ambienttemperature; U is a required driving temperature difference at a groundsurface between the ground and the circulation fluid in the second legat maximum heat power outlet from the ground at cool ambienttemperature; and L is a required driving temperature difference at theground surface between the ground and the circulation fluid in thesecond leg at maximum cool power outlet from the ground at warm ambienttemperature.
 2. The plant according to claim 1, wherein when keeping thewarm temperature in the at least one room, a temperature of thecirculation fluid fed to the at least one room temperature keepingdevice is higher than a room temperature, and a temperature of thecirculation fluid returning from the at least one room temperaturekeeping device is lower than the room temperature; and wherein whenkeeping the cool temperature in the at least one room, a temperature ofthe circulation fluid fed to the at least one room temperature keepingdevice is lower than a room temperature, and a temperature of thecirculation fluid returning from the at least one room temperaturekeeping device is higher than the room temperature.
 3. The plantaccording to claim 1, further comprising a device for complementaryaddition of energy which is connectable to the circulation circuit foraffecting the temperature of the circulation fluid fed to the energystorage unit.
 4. The plant according to claim 3, further comprising adevice for complementary energy supply which is connected to a feedingline from the energy storage unit to at least one room temperaturekeeping device.
 5. The plant according to claim 4, wherein thecirculation pump is arranged in an outlet line from the at least oneroom temperature keeping device, and an adjustable shunt line isarranged between a point downstream of the circulation pump and a pointupstream of the at least one room temperature keeping device.
 6. Theplant according to claim 3, wherein the circulation pump is arranged inan outlet line from the at least one room temperature keeping device,and an adjustable shunt line is arranged between a point downstream ofthe circulation pump and a point upstream of the at least one roomtemperature keeping device.
 7. The plant according to claim 1, furthercomprising a device for complementary energy supply which is connectedto a feeding line from the energy storage unit to at least one roomtemperature keeping device.
 8. The plant according to claim 7, whereinthe circulation pump is arranged in an outlet line from the at least oneroom temperature keeping device, and an adjustable shunt line isarranged between a point downstream of the circulation pump and a pointupstream of the at least one room temperature keeping device.
 9. Theplant according to claim 1, wherein the circulation pump is arranged inan outlet line from the at least one room temperature keeping device,and an adjustable shunt line is arranged between a point downstream ofthe circulation pump and a point upstream of the at least one roomtemperature keeping device.
 10. A plant for tempering a building,comprising: an in-ground energy storage unit formed by a plurality ofground heat exchangers coupled in parallel, each ground heat exchangerincluding a first tube-shaped leg surrounded by heat insulation and asecond leg in thermal connection with surrounding soil, the energystorage unit having a continuously varying temperature in a depthdirection between a cold end and a warm end; at least one roomtemperature keeping device for at least one room; a circulation pump forcirculating circulation fluid through a circulation circuit, whichincludes the circulation pump, the first and second legs of the groundheat exchangers, and the at least one room temperature keeping device,and in which the circulation fluid flows down one leg and up the otherleg in each of the ground heat exchangers; and a valve device forcontrolling a flow direction of the circulation fluid in the ground heatexchangers such that when keeping a warm temperature in the at least oneroom, circulation fluid is directed to the at least one room temperaturekeeping device from the warm end of the energy storage unit and whenkeeping a cool temperature in the at least one room, circulation fluidis directed to the at least one room temperature keeping device from thecold end, wherein the circulation pump is controlled such that atemperature difference S between an inlet and an outlet of the groundheat exchangers at all operating conditions has a constant value equalto 100% of a fixed value S_(MIN) for the plant, wherein S _(MIN) =F+U+Lwhere: F is a temperature difference between a required feedingtemperature to the at least one room temperature keeping device at amaximum warm keeping power at cool ambient temperature and at maximumcool keeping power at warm ambient temperature; U is a required drivingtemperature difference at a ground surface between the ground and thecirculation fluid in the second leg at maximum heat power outlet fromthe ground at cool ambient temperature; and L is a required drivingtemperature difference at the ground surface between the ground and thecirculation fluid in the second leg at maximum cool power outlet fromthe ground at warm ambient temperature.
 11. The plant according to claim10, wherein when keeping the warm temperature in the at least one room,a temperature of the circulation fluid fed to the at least one roomtemperature keeping device is higher than a room temperature, and atemperature of the circulation fluid returning from the at least oneroom temperature keeping device is lower than the room temperature; andwherein when keeping the cool temperature in the at least one room, atemperature of the circulation fluid fed to the at least one roomtemperature keeping device is lower than a room temperature, and atemperature of the circulation fluid returning from the at least oneroom temperature keeping device is higher than the room temperature. 12.The plant according to claim 10, further comprising a device forcomplementary addition of energy which is connectable to the circulationcircuit for affecting the temperature of the circulation fluid fed tothe energy storage unit.
 13. The plant according to claim 12, furthercomprising a device for complementary energy supply which is connectedto a feeding line from the energy storage unit to at least one roomtemperature keeping device.
 14. The plant according to claim 13, whereinthe circulation pump is arranged in an outlet line from the at least oneroom temperature keeping device, and an adjustable shunt line isarranged between a point downstream of the circulation pump and a pointupstream of the at least one room temperature keeping device.
 15. Theplant according to claim 12, wherein the circulation pump is arranged inan outlet line from the at least one room temperature keeping device,and an adjustable shunt line is arranged between a point downstream ofthe circulation pump and a point upstream of the at least one roomtemperature keeping device.
 16. The plant according to claim 10, furthercomprising a device for complementary energy supply which is connectedto a feeding line from the energy storage unit to at least one roomtemperature keeping device.
 17. The plant according to claim 16, whereinthe circulation pump is arranged in an outlet line from the at least oneroom temperature keeping device, and an adjustable shunt line isarranged between a point downstream of the circulation pump and a pointupstream of the at least one room temperature keeping device.
 18. Theplant according to claim 10, wherein the circulation pump is arranged inan outlet line from the at least one room temperature keeping device,and an adjustable shunt line is arranged between a point downstream ofthe circulation pump and a point upstream of the at least one roomtemperature keeping device.
 19. A method for tempering a building usinga plant for tempering the building, the plant comprising: (i) anin-ground energy storage unit formed by a plurality of ground heatexchangers coupled in parallel, each ground heat exchanger including afirst tube-shaped leg surrounded by heat insulation and a second leg inthermal connection with surrounding soil, the energy storage unit havinga continuously varying temperature in a depth direction between a coldend and a warm end, (ii) at least one room temperature keeping devicefor at least one room, (iii) a circulation pump for circulatingcirculation fluid through a circulation circuit, which includes thecirculation pump, the first and second legs of the ground heatexchangers, and the at least one room temperature keeping device, and inwhich the circulation fluid flows down one leg and up the other leg ineach of the ground heat exchangers, and (iv) a valve device forcontrolling a flow direction of the circulation fluid in the ground heatexchangers, the method comprising: controlling the valve device suchthat when keeping a warm temperature in the at least one room,circulation fluid is directed to the at least one room temperaturekeeping device from the warm end of the energy storage unit and whenkeeping a cool temperature in the at least one room, circulation fluidis directed to the at least one room temperature keeping device from thecold end; and controlling the circulation pump such that a temperaturedifference S between an inlet and an outlet of the ground heatexchangers at all operating conditions has a constant value equal to atleast 50% of a fixed value S_(MIN) for the plant, wherein S _(MIN)=F+U+L  where: F is a temperature difference between a required feedingtemperature to the at least one room temperature keeping device at amaximum warm keeping power at cool ambient temperature and at maximumcool keeping power at warm ambient temperature; U is a required drivingtemperature difference at a ground surface between the ground and thecirculation fluid in the second leg at maximum heat power outlet fromthe ground at cool ambient temperature; and L is a required drivingtemperature difference at the ground surface between the ground and thecirculation fluid in the second leg at maximum cool power outlet fromthe ground at warm ambient temperature.
 20. The method according toclaim 19, wherein the circulation pump is controlled such that thetemperature difference S between the inlet and the outlet of the groundheat exchangers at all operating conditions has a constant value equalto 100% of the fixed value S_(MIN).